The present invention relates to metallurgy, and more particularly to a method and apparatus for stirring molten metal directly in a furnace where the metal is melted, such stirring being advantageously effected to enhance the rate of melting or to maintain uniformity of composition or temperature in a standing body of molten metal.
A variety of methods have been used for stirring molten metal directly in the bath of a melting furnace, including mechanical, electromagnetic, gas dynamical and others. This invention proposes a method for stirring molten metals, such as aluminum and similar alloys, which is both effective and simple to carry out.
U.S. Pat. No. 4,008,884 discloses a method and apparatus for stirring molten metal. The apparatus described in this patent comprises a means for stirring a body of molten metal, including a tubular conduit structure arranged to project downwardly into the molten metal, and cooperating means (ejector) for effecting the withdrawal and delivery of metal through a nozzle at the lower end of the tubular structure. The procedure of stirring a body of molten metal comprises the steps of alternately withdrawing molten metal upwardly from the body in a confined space to a level above the body and expelling the withdrawn molten metal into the body as a submerged high velocity jet. The alternate metal-withdrawing and metal-expelling steps are effected by alternately applying suction and gaseous fluid (air) under pressure in the confined space above the molten metal body. The gaseous fluid (air) is supplied from a suitable source through solenoid inlet and outlet valves to the ejector.
The sequence of the metal-withdrawing and metal-expelling steps is controlled by means of a vacuum relay and by a time delay relay. To control a permissible level of the body of molten metal, an electrically conductive probe is introduced inside the stirrer pipe to signal and trigger a shutdown operation should metal rise into contact with the probe.
The method and apparatus of the patent referred to above have a number of disadvantages which, to a certain degree, limit the sphere of their application.
Immobile position of the stirrer pipe in the body of molten metal enables effective stirring of molten metal to be carried out in a rather limited area, which demands the installation of a great number of similar pumps, especially on large-capacity melting furnaces. In carrying out the melting of metal in a cylinder, - or square-shaped furnaces, even of relatively small capacity, the installation of at least two pumps will be required to ensure a rapid rate of melting or dissolving alloying additions. However, it is not always possible to mount a substantial plurality of pumps on a melting furnace, which in addition, requires an appreciable consumption of compressed gas. Furthermore, the withdrawal and expulsion of metal carried out at a definite constant level above the body of molten metal limits the sphere of application of the method and apparatus in question, which could have been rather promising from the point of view of creating optimum conditions for heat-and-mass exchange. This disadvantage is especially manifested in the course of melting solid charge, when the temperature of melt is still relatively low, and the washing of solid lumps of charge with a jet of molten metal having such low temperature is obviously insufficient to provide the best results insofar as the rate of melting and the use of the furnace heat are concerned. Furthermore, the use of the ejector for building up vacuum, which ejector is mounted on the pump and energized on completing the supply of compressed gas through a respective solenoid, drastically increases the time required for the metal-withdrawing operation. There seems to be hardly any apparent way to step up the rate of metal-withdrawing operation in this case also.
Under these circumstances, the operating efficiency of the pump is impaired, especially at the stage of melting solid charge, requiring a higher rate of the metal-withdrawing and metal-expelling operations.